Field of the Invention
The invention relates to an assembly for thermal insulation of a magnet in a magnetic resonance (MR) apparatus, in particular for thermal insulation at times during which the MR magnet is not used for MR tomography, e.g. during transportation thereof.
Description of the Prior Art
MR magnets in MR tomography apparatuses are typically designed as superconducting coil magnets. These are cooled by a coolant, typically helium, to superconducting temperatures. The coolant must be continuously cooled during operation, for which an electrically powered cooling unit (referred to as a “coldhead”) is provided.
MR tomography apparatuses can be transported to the target location with MR magnets that have not been cooled, and then first cooled to superconducting temperatures on site. Due to the time and the special equipment required for this, such on-site ramping up is complex and disadvantageous for the operator at the target location. For this reason, MR tomography apparatuses are often transported to the target location with MR magnets therein that are already cooled to superconducting temperatures.
If MR magnets are transported in an already-cooled state, a problem arises due to the cooling unit not operating during the transport. For this reason, it is impossible to prevent heating of the coolant during the transportation. Typical coolants (typically helium) vaporize during this heating, so that, depending on the extent of the heating, a more or less high loss of coolant occurs as a result of vaporization. The coolant losses can amount to as much as 50 liters/day.
The coolant reserve in an MR tomography apparatus is typically in the range of 1,200 liters. In order to prevent a complete evaporation of the coolant reserve, the transportation and installation at the target destination therefore must be completed within a maximum time period that must be strictly adhered to. Furthermore, the evaporated coolant must be replaced at the target destination, which results in additional logistical expenditures. Moreover, coolant, in particular helium, is relatively expensive, and thus a loss of coolant represents financial losses. For this reason, a reduction in the heating, and the thus resulting coolant losses, is desirable during such transport.
A cooling assembly having a cryostat is known from U.S. 2010/0041976 A1, in which inner and outer vacuum chambers are provided for the thermal insulation. The cryostat represents a heat bridge between the cooled interior and the environment. A ventilation structure enables the escape of vaporized coolant but the vaporized coolant is conducted by the ventilation structure such that the vessels and foils that are to be insulated are cooled by this vaporized coolant. By cooling the insulating structures, the thermal conductivity is reduced.
From U.S. 2012/0306492 A1 and U.S. 2012/0309630 A1, further assemblies are known for thermal insulation of cryostats. The assemblies are each based on a double-walled MR magnet container construction. The outer wall has an extended effective length for the thermal insulation. The inner wall is designed as a telescopic mechanism.
An MR tomography apparatus having superconducting magnets is known from U.S. Pat. No. 7,170,377 B2, with which reduced coolant losses occur. Vaporized coolant, normally helium, is cooled by a cooling unit, and re-condensed. The re-condensed coolant is returned to the cryostat. The cryostat is insulated by a radiation thermal shield and an additional super-insulating foil that acts against heating. The cooling unit, or the cryostat opening, in which it is disposed, is not likewise doubly insulated in its entirety, and therefore represents a relatively large heat bridge between the cooled interior and the environment.